Pigment grinding mill



July 4, 1967 J. D. POOTMANS PIGMENT GRINDING MILL 2 Sheets-Sheet 1.

Filed Oct. 31, 1965 v INVENTOR John D. POOTMANS ATTORNE]? y 4, 1967 J.D. POOTMANS 3,329,348

PIGMENT GRINDING MILL Filed 001:. 31, 1965 2 Sheets-Sheet 2 INVENTORJohn D. POOTMANS ATTORNEY United States Patent 3,329,348 PIGMENTGRINDING MILL John D. Pootmans, 322 Monmouth Ave., Mount Royal,Montreal, Quebec, Canada Filed Oct. 31, 1963, Ser. No. 320,432 Claims.(Cl. 241-30) This invention relates to a method of finely dividinggrinding or dispersing solids and to apparatus for carrying out thismethod which may be referred to as attritors.

The invention has particular application to grinding pigments, fillers,extenders, additives and other components. One use for example, is inthe grinding of pigments, and extenders in the manufacture of paints,and coatings, another in the grinding of iron compositions for coatingmagnetic tapes.

Various methods have been suggested for analogous purposes. Examples areshown in U.S. Patents 1,956,293, Kline et al., Apr. 24, 1934; 2,719,009,Szegvari, Sept. 27, 1955; 2,764,359, Szegvari et al., Sept. 25, 1956,and Canadian Patent 603,756, Szegvari, Aug. 23, 1960, the disclosures ofwhich are incorporated by reference. These methods use attritors whichinclude a stationary vessel containing a mass of spherical grindingballs and agitate the elements by an agitator passing through the middleof the mass.

The devices of the prior art and the methods which they perform whileeffective for the purposes they describe, sulfer from variousdisadvantages which will become clear from the following descriptionwhich compares them with the applicants method.

The present invention is based on a different type of agitation of thegrinding elements, which reduces the energy required, eliminates deadareas and reduces attrition of the vessel and grinding elements.Briefly, in the applicants method, agitating means is moved through theperiphery of the system close to the wall of the vessel at a speedsuflicient to impart flow substantially throughout the system and a flowpattern characteristic of the peripheral agitation. Depending upon thesize of the vessel, its shape, and the nature of the grinding problem,the agitating means may operate only at the periphery of the system orit may also operate within the mass. The agitation may also be modifiedby providing stator means which makes contact with the agitated grindingelements within the mass. It is also within the scope of the inventionto provide auxiliary agitating means, contacting the middle ofthe massof the grinding elements, which moves either in a counter direction orin the same direction at a different speed to the peripheral agitatingmeans.

A preferred form of vessel is cylindrical with vertical walls and eithera flat or a dished floor. In this case,

the agitator may take the form of an agitating shaft extending axiallyof the vessel with a horizontal arm extending from the bottom of theshaft substantially flush with the floor and a vertically extending armconnected to the end of the horizontally extending arm and just clearingthe side wall. A cover may be applied about the shaft to help preventlosses of volatile materials in the charge, and to reduce noise.

Other features will appear from the following detailed description whichwill be illustrated by reference to a preferred embodiment of theinvention which is shown in the accompanying drawings. In the drawings:

FIGURE 1 is a perspective view showing an attritor according to theinvention.

FIGURE 2 is a vertical cross-section as along the line 2--2 of FIGURE 1.

FIGURE 3 is a horizontal cross-section through the vessel as along theline 33.

FIGURE 4 is an enlarged fragmentary horizontal crosssection partly inplan through the wall of the attritor vessel just above the horizontalarm.

FIGURE 5 is a vertical cross-section to another form of attritoraccording to the invention.

FIGURE 6 is a fragmentary horizontal cross-section perspective view, asalong the line 66 of FIGURE 5.

Referring more particularly to the drawings, the attritor, in thepreferred form of the invention shown, is constructed as follows. It ismade up of an upright vessel of mild steel, having a cylindrical wall16, and a flat bottom or floor 18. Downwardly extending axially into thevessel A is an agitator shaft 17, on the lower end of which is mountedan agitator made up of a flat horizontal bar 19 of D cross-section. Thisbar just clears the bottom of the floor 18. This clearance is less thanthe radius of the smallest grinding element to be used. Upwardlyextending from the outer ends of the bar 19 are agitator blades 21 ofsubstantially the cross-section shown, which also operate just clear ofthe wall 16.

Extending upwardly from the bar 19 beside the respectiveblades 21 areoptional auxiliary blades 23 of rectangular cross-section. The vessel Ais provided with a discharge opening 31, which is controlled by a gatevalve 33, operated by an arm 35. The container A is filled with grindingballs, which reach substantially the level shown. Variations in thespecific characteristics of the various parts will be described later.

The blade 21 is provided at its trailing edge with a flexible sweepingmember or brush 24, held in a groove in the rear of the blade 121 by ascrew 26. The member 24 projects downwards beyond the lower surface ofthe blade 21 to engage the wall 16 and thus to sweep this wallcontinually, as the blade 121 rotates close to it.

The end of the member 24 is in the nature of a flap or scraper. It canbe a flexible metal sheet or of resilient material, say rubber orplastic.

FIGURE 6 is a cross-section perspective view somewhat similar to FIGURE4, showing an alternative arrangement of a scraping device for theagitator member 121. This scraping device is a strip 124 of flexiblematerial (similar to the member 24 of FIGURE 4), which is held to theupper surface of the leading face of the agitator member 121 by screws126. The scraper member 124 is arranged so that its edge projectsslightly beyond the lower surface of the agitator blade 121, so as toengage the wall 116 of the vessel and to sweep off any material adheringto the wall.

A crossbar C is shown extending across the top of the vessel A, althoughthere may be a cover, covering the entire top of the vessel, if desired.Mounted on the crossbars C is a superstructure which includes upwardlyextending arms 40 carrying a crossbeam 41 and further upwardly extendingarms 43 carrying a crossbeam 45. The agitator shaft extends up throughthe crossbar C and is provided with collars 47 and 49, adapted to bearagainst the crossbars C and 41 respectively. On its upper end, the

' shaft 17 carries a bevel gear 49 meshing with a bevel gear 51, mountedon a cross shaft 53, journalled in the arms 43. The shaft 53 is drivenby a pulley 55 through a belt 56, which is in turn driven by a suitablemotor which is not shown. This driving arrangement is calibrated toprovide the necessary power and speed in keeping with the desiredgrinding effect, as described herein, as will be understood by oneskilled in the art.

OPERATION In operation, the material to be ground is fed into the top ofthe vessel A and the agitator 17 set into motion so that the blades 21sweep just clear of the walls 16 at the periphery of the mass ofgrinding balls. This motion by the blades 21 and the bar 19 sets up acharacteristic form of peripheral agitation.

AGITATING MEMBER An attritor according to the invention will have atleast one agitating member of the following description. It will extendradially from the centre as a rigid bar along the bottom and the wall.It will match the inside shape of the vessel which Will thus be shapedas if engendered by a parallel member. This member will clear thevessels inner surface by less than the smallest radius of any grindingelement contained in the vessel so that no wedging occurs. It need notextend above the vessel wall. It should reach the highest level of theattritor charge under operating conditions. In this respect, it shouldbe noted that the level tends to rise at the periphery under centrifugalforce. In practice, to accommodate maximum charges, the member will beso constructed as to be about level with the top of the vessel wall.These overall dimensions of an agitating member of the main type may beexpressed as follows:

Internal radius of vesselradius of smallest grinding element radiallength internal radius of vessel.

Level of charge when in operationheightheight of vessel wall.

The motion of the main agitator member will be circular along theperiphery of the vessel. The rotational speed is preferably from about300 to about 900 feet per minute at the periphery. This represents for avessel of one foot diameter, an axial speed of about 100-300 r.p.m., orfor a vessel ten feet in diameter, an axial speed of about 30 r.p.m. Thespeed is actually not critical as in prior devices. In the patenteddevices mentioned, a minimum speed is needed for dynamic movement.Contrary to the devices based on so-called dynamic movement of thegrinding elements, which must operate above a certain speed threshold,any movement at all of the agitating elements in this invention sets upmovement and performs work throughout the charge.

An alternative form of attritor is shown in FIGURE 5. The same numbershave been used to denote similar parts, as in the previous figures, butthey have been raised by 100. In the device shown in FIGURE 5, a cover Chas been applied, or this can be merely a crossbar, as shown inFIGURE 1. On the cover or crossbar C are mounted downwardly extendingrods 122 in a position to lie between the upwardly extending peripheralagitator members 121 and the intermediate agitator members 123. Themembers 121 are static and intercept the mass which is being moved bythe members 121 and 123. The members 121 tend to prevent the mass ofgrinding balls and material being ground to move as a Whole, and causeit to be agitated internally, as it is brought into contact with themembers 121/122. In the form shown, the upwardly extending intermediateagitator members 123 are shorter than in the form of the invention shownin the previous figures, although this is not a necessary limitation.

The agitating members must be of rigid material suited to the product tobe ground. Mild steel, or mild steel clad with tetrafiuoroethyleneresin, sold under the name Teflon or other materials for specialpurposes are suitable. The shape of the bars themselves in cross-sectionmay be rectangular or rounded. A substantially D-shaped crosssection ofthe main agitating member with its flat surface facing the inside of thevessel along the bottom and against the wall, has been found helpful inpromoting additional desirable oscillating motions among the grindingelements. This type of cross-section is also more suitable than arectangular section from a hydrodynamic point of view, and reduces drag.The agitating members of the main type may be fitted with a feather edgeor scraper, for example, heavy gauge nylon or Teflon trailing or leadingin order to remove from the wall any material accidentally orcontinuously being splashed above the normal level. This scraperarrangement has been found to work best when fitted as a trailing edge,between the level of the charge and the top ofthe vessel wall, onto thearm of the main agitating member. A stator member is useful to reducethe tendency of a very thixotropic mass from being pushed around, as awhole and without internal agitation. The stator member tends to causeadditional internal agitation.

GRINDING ELEMENTS The grinding elements should be spherical and betweenabout Ma" and 2" in diameter. In a given vessel, they should all besubstantially of the same size. This size will be determined by theviscosity of the charge and the speed of rotation. A large size acquiresmore momentum than a smaller size and thus contacts between large sizegrinding elements are best able to shatter the particles to be ground,or dispersed, particularly at relatively high viscosities. On the otherhand, small grinding elements provide a greater number of contacts perunit of time and per revolution of the agitating members. The smallestsize that will suit the work to be done is to be preferred. Enoughelements should be used to fill the material to be ground. The grindingelements should be of a material at least as hard as the hardestmaterial to be ground. The more dense the better in order to preservemomentum. They must not be prone to shatter. Typical ball-mill elementsmay be used, for example, flint, stainless steel, porcelain or otherceramics.

NATURE OF THE AGITATION The agitation set up is what the applicant callsperipheral agitation. The term peripheral agitation as used in thisapplication, means the imparting of movement to a peripheral zone of thecharge of grinding balls and materials being ground (which is defined bythe wall of the containing vessel) whereby the grinding balls in thatzone are acted on by the agitating member. In the case of a cylindricalvessel the agitating member sweeps the side wall and floor and thus actsdirectly on the parts of the mass most difficult to energize. Thehorizontal parts cause an upward and outward movement which, in additionto providing grinding agitation, is most useful in sweeping the groundmaterials through the outlet. The vertical agitating parts causeoscillation of grinding elements in a horizontal plane which transmitsitself throughout the mass, the leading edge pushing the grindingelements towards the center and the trailing edge withdrawing them intoits wake. In vessels of large diameter, intermediate vertical agitatingparts reinforce the peripheral agitation. Stator parts, if used, willincrease attrition within the mass.

A circular pattern, in a horizontal plane, is set up by the main type ofagitating member. In addition, centrifugal action of the bar along thebottom drives material from the centre along the bottom towards thewall. This produces a continuous flow in a vertical circular patternwithin the peripheral agitation. Superimposed on this general movementof the charge is an oscillatory movement of the grinding elementsperpendicular to the surfaces of the agitating members whichcontinuously lift and push the grinding elements they contact. There areno dead spots. And, unlike the devices described in the above-mentionedpatents, the viscosity of the charge being worked does not reduce therange at which the agitators can impart motion to the grinding elements.The type of attritor in which the agitators do not reach the peripheryof the vessel suffer from severe limitations, since, as the viscosity ofthe charge goes up, the energy imparted to the grinding elements failsto reach the bottom or the walls. A recirculating pump is thus requiredin order to ensure even grinding in these devices. Another feature ofthe agitation set up by the applicants device is that the centrifugalforce along the bottom is sufficient to discharge the finished product,rendering dumpingand pumping arrangements unnecessary.

ADVANTAGES Among the advantages of the present invention are thefollowing.

( 1) Dead spots are less likely.

(2) In some of the prior art grinding methods, there are a number ofcritical factors which require special adjustment of the apparatus. Forexample, viscosity of the charge being ground, the movement of theagitators, the depth of the bed, and the clearance between the arms ofthe walls, all have to be coordinated. In the applicants type ofagitation, these factors are not critical because of the nature of theagitation and design of the agitator.

(3) In the prior art devices the agitators do not reach the periphery ofthe vessel. So as the viscosity of the charge goes up, the energyimparted to the grinding elements fails to reach the bottom or the wallsof the vessel. A recirculating pump is thus required to insure evengrinding or dispersion.

(4) In the applicants device, the centrifugal force along the bottom issufficient to discharge the finished product, This does away with thenecessity for special arrangements for pumping out the ground product.

(5) Some of the prior patents recommend that the drive shaft be operatedfrom worm gear so as to get a positive drive, necessary in order to movethe agitator against the heavy resistance encountered from theparticular agitating arrangement. This makes it impossible to turn theshaft manually and it is necessary to resort to complicated andexpensive coupling devices.

(6) In other devices, extreme wear occurs at the extremities of theagitating arms because of their high speed through the grindingelements. This requires expensive hard alloys for the agitators. In theapplicants device, the speed of the agitating elements is lower relativeto the grinding elements. Remarkably little wear occurs. In acylindrical vessel, it affects mainly the vertical arms. No wear of amild steel vessel wall was noticed after hundreds of hours of operation.

(7) Since discharge in the applicants device is effected or facilitatedby the manner of agitation, the apparatus does not have to be uncoupledfrom the drive for dumping. The device may be fixed to the floorpermanently.

(8) The method of the invention can handle paint materials in the wet,for example, alkyd resin with a vehicle and pigments in it, water andpigments, solvents and vehicles, pigments and/or fillers, or any othergrinding job within the paint industry.

EXAMPLES The invention will be further explained by reference to thefollowing examples. These examples are carried out on a typical attritorincluding a vessel of the type shown on the drawings, of about 4 /2 feetin diameter and 2 /2 feet high. In this device, the bar 19 was about 4inches in width by about 2 inches thick. Blades 23 were about 3 /2inches wide and 1 /2 inches thick. The range of rotational speed for theshaft 17 which is found effective, was from about 20 to about 80 r.p.m.or a linear speed the periphery of around 700 feet per minute. Theapparatus had a capacity of about 60 gallons. It was driven by a 5 HRelectric motor through a belt drive.

Example 1 A run was carried out in a vessel of the type described in theprevious paragraph and it is of the nature of the agitating elements andof the charge, as shown. The amount of the charge, temperature and timeof grinding as well as the fineness after various grinding periods isindicated.

A similar grinding operation as in Example 1, was carried out, accordingto the following criteria.

Charge Lbs. Temp; Time Hegrnan, N.S.

Fineness 1 Selected Flint Pebbles. Carbon Black Witco FL. 1 hr 2 EpoxyEster Soln., 60%.. 2 hrs.- 4 Ca Naphthenate 6%. 3 hrs. 5 Pb Naphthenate24%.... 4 hrs 5 Example 3 A still further grinding operation is carriedout as follows, with the results shown.

Charge Lbs. Temp. Time Hegman, N.S.

Fineness V Steel Balls 1 hr 4 Carbon Black Witco Fl as above 2 hrs 6Epoxy Ester as above. 3 hrs 7 Example 4 A grinding operation which iscarried out using the following amounts and with the following results.

Charge Lbs. Temp. Time Hegman, N.S.

Fineness Chrome Yellow Deep,

MeArthur Irwin 320 Toluidine Red Medium,

Dominion C01 40 Alkyd, 65% Soya, 70%

in Min. Spirits 300 Using Pebbles, 1--.... 300 8 hrs.-.. 4 Using,instead of Alkyd:

Chlorinated Rubber, 10 cps Xylene 150 8 hrs 3 6 Latter film was weak,possibly due to polymer breakdown under attrition. Recommend grinding inplasticizer and solvent instead of in the chlorinated rubber.

VESSEL The vessel may be of various shapes. For example, in thepreferred form shown, it has a cylindrical wall and a floorsymmetrically related thereto. The floor can be flat or dished.Likewise, the vessel can have a hemispherical wall or a wall of someother shape whose surface always lies on a circle of which the agitatingmem ber shaft is at the centre so that an agitating member mounted onthe shaft is always equidistant from the wall.

I claim:

1. A method of grinding and of dispersing by means of an attritor inwhich a mass of substantially spherical attritive grinding elements iscontained to a substantial depth along with a material to be ground in avessel having a confining wall, comprising moving an agitating blade ina peripheral direction and close to said wall throughout the depth ofsaid grinding elements through a major part of the periphery of the massat a distance from the wall less than'the radius of the smallestgrinding element at a speed effective to impart flow throughout the masswhile leaving radially inward portions of the mass free from contactwith said agitating blade.

2. A method of grinding, as defined in claim 1, in

which the agitating blade is moved at a linear speed at the periphery ofthe mass within the range from about 300 to about 900 feet per minute.3. A method of grinding, as defined in claim 1, in which the mass isalso subjected to the action of a blade acting on the mass in a relativedirection counter to that in which the said agitator is moving.

4. A method, as defined in claim 1, in which an agitating blade is movedthrough a middle part of the mass in a direction counter to the movementof said peripheral agitator.

. 5. In a device of the type described, a vessel adapted to contain amass of grinding balls and material to be ground and a 'blade adapted topass about the surface of said vessel thereby to provide peripheralagitation for the mass, said blade being spaced from the wall a diameterless than the distance of the grinding balls to be used and provided atone edge thereof with a flexible sweeping member projecting beyond-thesurface of the blade into contact with the wall of the vessel.

References Cited UNITED STATES PATENTS 1,001,851 8/1911 Hunter 259-1051,430,070 9/1922 Franzwa 259-107 1,854,732 4/1932 Beran 259-1072,592,994 4/ 1952 Ahlmann 241-172 XR 2,595,117 4/1952 Ahlmann 241-172 XR2,858,861 11/1958 Appleton 259-105 XR 3,054,565 '9/ 1962 Willems 259-107XR 3,199,792 8/1965 Norris 241-172 XR FOREIGN PATENTS 82,001 11/1934Sweden.

WILLIAM W. DYER, JR., Primary Examiner.

HARRY F. PEPPER, JR., Examiner.

1. A METHOD OF GRINDING AND OF DISPERSING BY MEANS OF AN ATTRITOR INWHICH A MASS OF SUBSTANTIALLY SPHERICAL ATTRITIVE GRINDING ELEMENTS ISCONTAINED TO A SUBSTANTIAL DEPTH ALONG WITH A MATERIAL TO BE GROUND IN AVESSEL HAVING A CONFINING WALL, COMPRISING MOVING AN AGITATING BLADE INA PERIPHERAL DIRECTION AND CLOSE TO SAID WALL THROUGHOUT THE DEPTH OFSAID GRINDING ELEMENTS THROUGH A MAJOR PART OF THE PERIPHERY OF THE MASSAT A DISTANCE FROM THE WALL LESS THAN THE RADIUS OF THE SMALLESTGRINDING ELEMENT AT A SPEED EFFECTIVE TO IMPART FLOW THROUGHOUT THE MASSWHILE LEAVING RADIALLY INWARD PORTIONS OF THE MASS FREE FROM CONTACTWITH SAID AGITATING BLADE.